Relationships of milk culture status at calving with somatic cell counts and milk production of dairy heifers during early lactation on a Californian dairy

Four-quarter, composite milk samples were collected from 339 heifers calving for the first time in a large Californian dairy which consistently had low herd somatic cell counts and low prevalence of major mastitis pathogens. The milk samples were collected on average at 6.4 days post partum (range 1-17). Thirty-nine percent of the heifers were subclinically infected with coagulase-negative Staphylococcus spp. Other results were: no growth, 29%; coliform only, 16%; Streptococcus spp. only, 11%; Streptococcus spp. and coagulase-negative Staphylococcus spp. only, 4%; others, 1%. The somatic cell counts and milk production were not significantly different (P > 0.05) between culture groups over the first five monthly Dairy Herd Improvement test periods. Somatic cell counts decreased significantly after the first test period within the Streptococcus spp. group (P < 0.002) and Staphylococcus spp. group (P < 0.01). Milk production was higher during most of the second and third test periods (P < 0.03) compared with the first period. The frequency of heifers lost to analysis was not different between the culture groups. Results from this dairy indicate that subclinical infections with minor pathogens (primarily coagulase-negative Staphylococcus spp.) had no significant effect on average somatic cell counts or milk production during early to middle lactation.